The invention relates to a method defined in the preamble of claim 1 for distributing the capacity of a transmission system in a base station network, which method makes it possible to increase the number of base stations using a given transmission system and/or improve the utilization rate of the transmission system. The invention also relates to a transmission system applying such a method.
Communication between a base station controller (BSC) and the base transceiver stations (BTS) controlled by it in a GSM (Global System for Mobile telecommunications) network, for example, are usually arranged as follows: Transmission is realized using bi-directional time-division-based 2-Mbps systems. A system includes thirty-two 64-kbps time slots each of which can be divided into four 16-kbps partial time slots. One (point-to-point) or several base transceiver stations may be connected to such a system in a chain, multidrop, loop or star configuration. Base transceiver stations have one or more transmitter/receiver (TRX) units which comprise eight 16-kbps bi-directional traffic channels (TCH). For each TRX unit the transmission system allocates in a fixed manner two time slots for traffic channels and one 16-kbps partial time slot for TRX signalling (TRXSIG). In addition, the system reserves in a fixed manner for each base transceiver station one 16-kbps partial time slot for operation and maintenance unit signalling (OMUSIG). Thus, one transmission system suffices for 12 TRX units. In this maximum case only a few partial time slots are left unused; the exact quantity depends on how many base transceiver stations the TRX units are divided into. There are also arrangements in which the traffic of 10 TRX units is placed in the 2-Mbps system. Furthermore, there are arrangements in which some of the transmission system time slots contain GSM traffic and some contain NMT (Nordisk MobilTelefon) traffic or the traffic or paging traffic of some other cellular radio system.
The method according to the prior art and a system embodying it are disclosed e.g. in a Nokia Telecommunications document xe2x80x9cTRUA Base Station Transmission Unit, Product Descriptionxe2x80x9d, NTC C33315002SE_B0, Nokia Telecomnunications Oy 1995-1996. FIG. 1a shows a possible base station network using a 2-Mbps transmission system. The base transceiver stations in it are chained through a cable originating from a base station controller 101. Base transceiver station BTS1 (102) has six TRX units sectored e.g. in such a manner that each of the three sectors has two TRX units. Base transceiver stations BTS2, BTS3 and BTS4 each have two omnidirectional TRX units. The interface unit 103 in base transceiver station BTS1 connects in bi-directional manner time slots T1-T12 to the radio channels of the TRX units. Placement of traffic channels TCH in the time slots is shown in more detail in FIG. 2. Rows in the table correspond to time slots T0-T31 and X indicates that the partial time slot in question is unused. Similarly, base transceiver station BTS2 reserves time slots T13-T16, BTS3 time slots T17-T20, and BTS4 time slots T21-T24.
Separate time slots must be allocated for signalling (TRXSIG) and operation and maintenance (OMUSIG). In the exemplary case, base transceiver station BTS1 uses for these purposes time slots T25, T26 and T27, BTS2 uses time slot T28, BTS3 time slot T29 and BTS4 time slot T30 in accordance with FIG. 2.
The base station network shown in FIG. 1a has a chain topology. In FIG. 1b, the base station network has a star topology as a connection is branched from base transceiver station BTS3 to two other base transceiver stations BTS5 and BTS6. TRX units are distributed between the base transceiver stations slightly differently from FIG. 1a in order to keep their total number the same. In FIG. 1c the base station network has a loop topology as a direct communications connection is provided between base transceiver station BTS4 and the base station controller BSC. The loop topology is used in prior-art base station networks mainly for securing communications as in this configuration all base transceiver stations in the base station network have two alternative communications connections with the base station controller (the alternative connections are in the opposite directions of the loop formed by the base transceiver stations). FIG. 3 schematically illustrates a base transceiver station 300 in such a loop-configured base station network. Communication between the base transceiver station 300 and other apparatus in the same base station network takes place through a transmission unit 301 (TRU). The transmission unit 301 is a cross-connect in which a certain branching table (not shown) determines how the various time slots are connected straight through the transmission unit 301 from left to right (or from right to left) and which time slots are connected via the lower part of the transmission unit 301 to the base transceiver station control functions (BCF) part. Through the latter, the transmission capacity represented by the time slots is distributed between the TRX units 302 and 303 of the base transceiver station. In accordance with the usual practice, FIG. 3 shows the various time slots as separate signal lines although in reality they are transferred via the same physical connection. This example assumes that six time slots are connected straight through the transmission unit 301 (lines 304) and two time slots are connected to the base transceiver station""s TRX units 302 and 303 (lines 305 and 306).
In FIG. 3 the transmission unit 301 comprises two so-called Y-type protection switches 307 and 308 by means of which the system utilizes the loop topology of the base station network. The transmission unit monitors the so-called pilot information accompanying the signals coming from the different transmission directions and determines whether the time slots used by the base transceiver station""s TRX units 302 and 303 should be routed via the left-hand-side path or via the right-hand-side path between the base transceiver station and base station controller. In FIG. 3, the transmission unit has detected that the time slots represented by lines 305 and 306 should be transmitted via the left route, so the protection switches 307 and 308 have been set so as to connect the base transceiver station""s TRX units 302 and 303 to the left branches of lines 305 and 306. In case of a different measurement result one or both of the protection switches 307 and 308 could be set into the other position indicated by the broken line, in which case the traffic in the time slot in question would be routed via the right-hand-side path in the base transceiver station 300. Setting of the protection switches 307 and 308 is realized such that a change is made in the current branching table in the transmission unit 301. It is obvious that in this context the directional terms xe2x80x9cleftxe2x80x9d, xe2x80x9crightxe2x80x9d and xe2x80x9cdownxe2x80x9d only refer to the orientation shown FIG. 3 and bear no relation to the actual situation.
A disadvantage of the present method is that the transmission system reserves capacity for the base transceiver stations"" TRX units according to the maximum traffic, regardless of the actual traffic situation. Thus, at times, the network operator has to pay for unnecessary transmission capacity. A further disadvantage of the present method is that if additional mobile communications capacity has to be built in a given area to such an extent that the number of TRX units exceeds 12, the operator has to provide a new, even more underutilized transmission system.
The object of the invention is to reduce the disadvantages mentioned above. The method according to the invention is characterized by what is expressed in the independent claims.
The basic idea of the method is as follows: At least part of the time slots in the transmission system are shared by the base transceiver stations and their TRX units. A given time slot or partial time slot can at different moments be allocated to different TRX units according to the traffic situation. Some of the traffic time slots are allocated to the TRX units in a fixed manner and the rest are shared, or all time slots are shared. In the latter case, too, it is preferable to allocate fixed partial time slots for TRX signalling (TRXSIG) and operation and maintenance signalling (OMUSIG).
It is thus an advantage of the invention that the capacity of the transmission system can be utilized more efficiently, because it can always be directed to those TRX units, base transceiver stations and areas which have the most traffic. Compared to the current practice, more TRX units can be attached to the transmission system. This is significant, especially in the case where the network operator has to lease the transmission connections. Consequentially, it is a further advantage of the invention that as the amount of traffic increases in a given area, the introduction of a new transmission system can be postponed, as compared to the current practice.